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Comparing coeval plutonic and volcanic rocks in the Aleutian arc:
Are primitive, mafic lavas representative of arc fluxes?
Peter Kelemen, Sam Bowring, George Gehrels, Steve Goldstein, Mike Gurnis, Brian Jicha, Bob Kay,
Suzanne Mahlburg Kay, Mike Perfit, Matt Rioux, Dave Scholl, Tracy Vallier and Gene Yogodzinski
Studies of geochemical cycling in subduction systems commonly assume that primitive basaltic
magmas are representative of the compositional flux through the arc Moho, and/or of the bulk
composition of arc crust. These assumptions are rarely tested. The Aleutian arc is unique among intraoceanic arcs in its widespread exposure of Paleogene and Neogene, mid-crustal, felsic plutonic rocks, as
well as their host lavas. Preliminary data suggest that many Aleutian plutonic rocks are derived from
parental magmas that are geochemically distinct from typical basaltic lavas in the arc (Figure 1), perhaps
because relatively hydrous magmas degas and stall in the mid-crust. If so, mafic lavas might not be
representative of arc crustal bulk composition, or of net magmatic flux through the Moho into arcs. In
order to evaluate this hypothesis, and a host of other fundamental questions, it is necessary to make a
systematic comparison of the composition of coeval Aleutian plutonic and volcanic rocks.
Continental crust has been generated via geochemical processes similar to arc magmatism, perhaps
followed by later reworking of arc crust. However, arc lavas worldwide are dominantly “mafic”, or
basaltic, while continental crust is “felsic”, with an andesitic or dacitic bulk composition. Also, it is often
inferred that bulk arc crust is mafic, on the basis of dominantly basaltic lavas, and high lower crustal
seismic P-wave velocities in some arcs. As a consequence, petrogenetic processes have been proposed to
produce felsic crust from a mafic protolith, including (1) formation of a felsic mid-crust via magmatic
differentiation of basalt, followed by (1a) “delamination” of dense, mafic or ultramafic lower crust, or
(1b) subduction and then “relamination” of buoyant, felsic mid-crustal rocks during subduction erosion
and arc-arc collisions. Alternatively, (2) mid-crustal plutons, or entire arc sections, may be derived from
mantle-derived andesitic magmas, rather than from the basaltic magmas common among lavas.
Notably, recent seismic data on the Izu-Bonin-Mariana (IBM) arc, together with reconstructed arc
seismic sections for the Jurassic Talkeetna arc and the Jurassic-Cretaceous Kohistan arc, all suggest that
these intra-oceanic arcs have a relatively felsic bulk composition, at least above the seismic Moho.
Perhaps (as in hypothesis 1), all three arcs underwent substantial modification by delamination, or (as in
2) voluminous, early arc magmatism included a large proportion of primitive andesite. And, in the case of
IBM, perhaps mafic to ultramafic cumulates are still present below the Moho. Seismic velocities for
Aleutian lower crust appear to be higher than for IBM, but interpretation of these data is complicated by
the unusual nature of the two arc crossings, and the oblique fore-arc to arc geometry of the one strike line.
In any case, our focus here is on the plutonic middle crust.
Systematic study of coeval felsic and mafic rocks in an intra-oceanic arc could provide the essential
information needed to unravel these different hypotheses. For example, (1) suggests that there should be
no systematic difference in radiogenic isotope ratios between felsic mid-crustal plutons and coeval mafic
lavas, since both are derived from the same mantle source. Alternatively, systematic differences between
felsic plutons and mafic lavas would support hypothesis (2). This is crucial, since (2) suggests that
primitive basalts are not representative of the net magmatic flux through the Moho to form arc crust.
Furthermore, understanding the genesis of felsic plutons that are coeval with dominantly basaltic
lavas can provide fundamental insight into the processes of arc crustal accretion, regardless of whether
felsic plutons are differentiated from typical arc basalts or not. In one view, high temperature, low-H2 O
mafic melts with low viscosity erupt readily, whereas lower temperature, higher-H2 O felsic magmas
undergo degassing in the mid-crust, and become too viscous to ascend further. In order to understand arc
magmatism, it is essential to test this hypothesis, and quantify the nature of any systematic bias arising
from such physical processes. For example, studies of H2 O-contents in melt inclusions in lavas might not
yield an unbiased estimate of H2 O contents in the magmas that form plutons.
Throughout most intra-oceanic arcs, felsic mid-crustal rocks are not exposed at all. The Miocene,
tonalitic Tanzawa plutonic complex in Japan is inferred to be tectonically exposed, felsic mid-crust from
the IBM arc. However, it is geochemically distinct from continental crust (e.g., Tanzawa is depleted in K
and light REE, whereas CC is enriched), and lacks spatial and temporal context with the rest of the arc.
In contrast, the Aleutian chain has characteristics that make it ideal for a study of plutonic rocks in an
intra-oceanic arc. (A) The Aleutians have never been rifted, and still contain strata recording ~ 40 Ma of
arc history. (B) Some primitive lavas have Nd, Sr, Pb and Hf isotope ratios indicating a depleted upper
mantle source, similar to the MORB source. These are the isotopically depleted end-member among arc
lavas worldwide. They are dominated by juvenile igneous material, rather than recycled components from
continental crust and terrigenous sediments. (C) Despite their lack of recycled, older continental material,
these same primitive Aleutian lavas have compositions almost identical to bulk continental crust, more so
than in any other intra-oceanic arc. Formation of juvenile igneous rocks with the composition of
continental crust is occurring in the Aleutians today. (D) Intrusive rocks – predominantly quartz diorite to
granodiorite – are exposed on many of the larger islands together with their host volcanic rocks. The
widespread presence of exposed intrusions in the Aleutians provides an unmatched opportunity for direct
study of mid-crustal plutonic rocks, and their relationship to coeval volcanics.
We propose an extensive study of Paleogene and Neogene plutonic rocks and coeval volcanic rocks,
together with volcanoclastic rocks in the Aleutians. We need to compare samples from the same island
that have similar ages, so an important secondary outcome of our study will be extensive data on the
geochemical evolution of the arc over time. Volcanic and plutonic samples will undergo zircon and
40
Ar/39Ar geochronology, XRF and ICP-MS geochemistry, and radiogenic isotope analyses, and we will
undertake geochemical and detrital zircon studies on volcanoclastic rocks.
Preliminary analytical work can be done on existing samples from [a] relatively detailed studies
(Captains Bay pluton, Unalaska Island; Hidden Bay and Finger Bay plutons, Adak I.; Kagalaska pluton,
Kalalaska I.), [b] reconnaissance mapping (large plutons other than Captains Bay on Unalaska I., southern
parts of Atka I., Umnak I., Amchitka I., Attu I., Amlia I., Komandorksy Is.), and [c] dredging and
submersible studies south of Adak and Kiska I. These will provide ages – including ages of detrital
zircons in volcanoclastic rocks – to extend previous 40Ar/39Ar work, and geochemical data for initial
constraints on the extent of isotopic variability within and between plutonic and volcanic suites.
Following these initial studies, we propose to conduct field work on several islands containing a
variety of plutons of varying ages, together with their older volcanic host rocks and younger, overlying
volcanics. Because Adak is relatively well-studied, the best targets seem to be the southern part of Atka,
where excellent reconnaissance mapping suggests great potential, and the relatively accessible plutonic
rocks on Unalaska and Umnak. Away from Unalaska, outcrops are mainly on sea cliffs along the shore.
Depending on the level of funding, this field work can be conducted via Zodiak, or – preferably – with
helicopter support from a research vessel such as the Maritime Maid (http://www.maritimehelicopters.com/).
To expand our spatial and temporal coverage, we will propose separate dredging and/or submersible
studies of steep topography in the fore-arc. (The oldest known sample from the Aleutian arc is a plutonic
rock from Murray Canyon, south of Kiska I). And, we will seek continuing collaborations with Russian
colleagues to continue studies of Paleocene to Eocene volcanoclastic arc rocks (Aleutian? pre-Aleutian?)
in the Komandorsky Islands, with the understanding that we would be happy to assist in sample analyses.
Our study will provide crucial information on mid-crustal rock compositions, together with the extent
of fracturing and metamorphism, which can be used in interpreting existing and proposed, new seismic
data on the Aleutian arc. Similarly, our petrological studies will provide constraints on the nature of
deeper plutonic rocks in the middle and lower crust, which can be compared to inferences from seismic
investigations in a dialectical process that will refine our understanding of arc lower crust.
Figure 1: Average Aleutian felsic plutons (> 55 wt% SiO2) compared to average lavas and average felsic lavas (> 55
wt% SiO2). See Kelemen et al., AGU Monograph, 2003 and Treatise on Geochemistry, 2003 for data sources. Felsic
plutons are enriched in some incompatible elements, compared to lavas, indicating that the plutons probably
represent liquid compositions. The large difference at the same SiO2 (and at the same molar Mg/(Mg+Fe) or Mg#,
not shown) suggests that the plutons were not derived by crystal fractionation from the same parental magma
composition as the lavas. Either (1) mixing of evolved and primitive compositions was systematically more
important in forming the plutons, or (2) the plutons had a parental melt that was systematically different from the
parent for the lavas. The data summarized here are mainly for Holocene lavas and Miocene plutons, so it is not yet
clear whether the differences reflect spatial or temporal variation in magmatic processes.
Figure 2: Comparison of average composition for Aleutian felsic plutons (> 55 wt% SiO2; compilation in Kelemen
et al., 2003) with bulk continental crust (Rudnick & Gao, Treatise on Geochemistry, 2003).